An inkjet printing apparatus forms an image by fixing small ink droplets serving as a coloring material onto the surface of a printing medium. Recently, printing is done on a printing medium by using not only four conventional color inks including cyan (C), magenta (M), and yellow (Y) color inks and black (Bk) ink, but also low-density inks of similar colors (e.g., light magenta and light cyan), and orange, blue, green, and skin color inks.
The volume of one ink droplet used in the inkjet printing apparatus decreases to 1.0 pl (picoliter) in order to meet recent demands for higher image quality.
An ink droplet 1.0 pl in volume is regarded as mist, and it becomes difficult to control ink droplets in such a small volume one by one.
From the viewpoint of high printing quality, it is desired to attach droplets of, e.g., 1.0 pl or less onto desired positions on a printing medium at a precision of micron order. However, it is difficult to obtain the desired precision under the influence of a peripheral air flow. Immediately after discharging ink, fine ink droplets called “satellites” which are produced when originally one ink droplet is broken into a plurality of ink droplets may attach to unintended positions or float in space.
For this reason, it is difficult to accurately attach all droplets to desired printing positions.
If the above-mentioned satellites or ink droplets bounded back from the surface of a printing medium float in the air to accumulate fine ink droplets, the fine ink droplets contaminate the interior of the printing apparatus to degrade the movable characteristic of the movable portion of the printing apparatus. In addition, fine ink droplets cause various sensors to malfunction, or gathered floating mist attaches to the upper surface of a printing medium or the backside of the next printing medium to contaminate it.
In order to solve this problem, there has conventionally been proposed a method of charging ink droplets and controlling them in an inkjet printing apparatus.
For example, in Japanese Patent Publication Laid-Open No. 5-008392, the electric field is controlled to be applied between a printhead and a printing medium and to be stopped during ink discharge. This control prevents positive or negative charging of ink droplets by the electric field and a failure of ink charged to either polarity in attaching to a printing medium.
Japanese Patent Publication Laid-Open No. 5-104724 proposes a method of injecting charges into ink in the printhead and attracting ink toward a printing medium.
Japanese Patent Publication Laid-Open No. 5-124187 proposes a method of controlling the electric field and discriminately controlling main droplets and subsequent satellite droplets.
Japanese Patent Publication Laid-Open No. 2002-211005 proposes a method of positively or negatively charging each of plural types of inks and capturing mist by an electrode.
Japanese Patent Publication Laid-Open No. 2003-014773 proposes a method of charging ink by an ionizer and collecting ink droplets.
The techniques disclosed in these prior arts have the following problems.
In order to implement high-speed printing, control of the electric field according to Japanese Patent Publication Laid-Open No. 5-008392 must be performed at a very high frequency. It is practically difficult to perform such control, or high-speed printing is limited. Electromagnetic waves are generated by high-frequency control of the electric field and act as a noise source, degrading the reliability and safety of the printing apparatus.
In the method according to Japanese Patent Publication Laid-Open No. 5-104724, polarization occurs because, when a fine droplet is discharged from the printhead, it elongates in the discharge direction and is broken into a plurality of droplets. Upon polarization, a fine droplet is charged positively or negatively. A fine droplet may be attracted to a printing medium or repulsed by the printing medium. It is difficult to control a fine droplet.
In the method according to Japanese Patent Publication Laid-Open No. 5-124187, polarization as described above occurs, and separation of satellite droplets slightly changes one by one. It is, therefore, difficult to accurately control a satellite droplet.
In the method according to Japanese Patent Publication Laid-Open No. 2002-211005, the structure of the printing apparatus becomes complicated because a charging mechanism for each type of ink must be arranged.
The method according to Japanese Patent Publication Laid-Open No. 2003-014773 does not intend to force ink droplets to move toward a printing medium, and poses a problem in achieving high-quality printing.